1. Field of the Invention
The invention relates to a transmission system comprising a data transmitter, for applying data symbols to an input of a channel at a symbol rate, and a data receiver. The data receiver comprises deriving means for deriving a detection signal from an output signal of the channel, and a detection means for deriving a sequence of data symbols carried by the detection signal from N an integer number values of the detection signal per symbol interval. The detection means includes deriving means for deriving a reliability measure which is representative of the reliability of the sequence of data symbols derived from the detection signal.
The invention likewise relates to a receiver for such a system.
2. Related Art
A system as defined in the opening paragraph is known from the journal article "On Optimal Detection of Band-Limited PAM Signals with Excess Bandwidth" by G. M. Vachula and F. S. Hill in IEEE Transactions on Communications, Vol. COM-29, No. 6, June 1981, pp. 886-890.
Such a transmission system may be used, for example, for transmitting data signals over the public telephone network or for reconstructing data signals coming from a magnetic tape or disc. When data symbols are transmitted via a transmission medium or when data symbols are stored on a recording medium respectively, the data symbols to be transmitted or recorded respectively, are converted into analog pulses to be subsequently applied to the transmission medium or recording medium respectively, to be denoted hereinafter by the term channel.
In the receiver the output signal of the channel is generally filtered and then sampled at a sampling rate equal to the symbol rate 1/T, as a result of which a detection signal is obtained. If the bandwidth of the filtered output signal of the channel exceeds half the symbol rate value, a sampling rate higher than the symbol rate 1/T is necessary according to the Nyquist sampling theorem to avoid information present in the filtered output signal getting lost. This information getting lost may be shown by an enhanced symbol error rate when the sequence of data symbols is approximated from the detection signal.
In order to avoid information in the filtered output signal of the channel getting lost, a sampling ram which is a factor N higher than the symbol rate is used in the receiver known from above journal article. As a result, N different values of the detection signal per symbol interval are available for determining the sequence of data symbols carried by the detection signal.
For determining the maximum likelihood sequence of data symbols, a reliability measure, indicating how reliable the already determined data symbols are, is used in the prior-art transmission system. In the prior-art transmission system this is effected by means of a Viterbi detector. The detection signal is then sampled twice per symbol interval. There is thus a first sample and a second sample of the detection signal available per symbol interval. The reliability measure is proportional to the sum of the difference between the current and expected values of the first sample and also to the difference between the current and expected values of the second sample of the detection signal.
It has appeared that in such transmission systems the symbol error rate at a specific signal-to-noise ratio of the detection signal is still larger for some channels than may be expected with an optimum system.